Organic electronic devices have attracted considerable attention since early 1990's. Examples of organic electronic device include Organic Light-Emitting Diodes (OLEDs), which include Polymer Light-Emitting Diodes (PLEDs) and Small Molecule Light-Emitting Diodes (SMOLEDs). Due to the sensitivity of the organic materials used in organic electronic devices, such as charge transport layers, electroluminescent material, and the cathode to moisture and oxygen, organic electronic devices typically have rigorous package requirements for practical applications. Two types of packaging structures adopted for organic electron devices include (1) a sealing can with air gap and solid-state desiccants or epoxy with glass cover sheets and (2) a thick metal solder layer directly attached to a common cathode.
In active matrix (“AM”) driven devices, the common electrode layer can be subjected to high current density when the display size is large or when the emission intensity increases above certain levels. In such cases, if the heat flowing from the light emitting junction area to the ambient air is insufficient, a significant temperature rise will take place in the device. The operation lifetime of OLEDs is strongly dependent on operation temperatures. For poly (phenylenevinylene) (PPV) derivatives with yellow or orange colors, the operation lifetime can be approximately 35 times shorter at 80° C. than at 25° C. (following a thermally activated form of:τ1/2(T)=τ1/2(T0)*e(−T/T0) wherein τ1/2(T) and τ1/2(T0) are half lives of an organic active material at temperatures of T and T0, respectively). This implies that the operation lifetime can be reduced by approximately a factor of two for each 5° C. increase in temperature near room temperature.
The EL efficiencies of OLED pixels are typically in range of approximately 1-20 cd/A. For a full-color display with 50% aperture ratio and with a circular polarizer with approximately 40% optical transmission, the pixel current density is in range of approximately 30-600 mA/cm2 for an emission intensity of approximately 400 cd/m2. At an operation voltage of approximately 5 V, the corresponding input electric power density is in range of 0.15-3.0 W/cm2.
FIG. 1 includes an illustration of a conventional OLED including a common cathode 166 that is covered by an epoxy layer 182 and glass sheet 184. FIG. 2 includes an illustration of another conventional OLED that is covered by a metal cap 284 having a desiccant 286. The metal cap 284 is attached using an adhesive 282. The OLEDs and their formation are described in more detail later in this specification (Control Examples within the Examples section). The OLEDs in FIGS. 1 and 2 may have thermal resistance coefficients (defined later in the specification) that are typically greater than 150° C.·cm2/W. The corresponding temperature rise of the emission pixels could be higher than 10° C. when operating at 200 cd/m2 for a display having an area of approximately 3-6 cm2.
For some outdoor display applications, an intensity of 500-2000 cd/m2 is used. Organic electronic devices that can be used for lighting panels may have an emission intensity of 2000-5000 cd/m2 in order to replace conventional fluorescent lights. AM-driven OLED displays and lighting panels with encapsulation schemes, such as the ones shown in FIGS. 1 and 2, may not be stably operated at such high brightness levels, and the device temperature may not even be stabilized due to insufficient heat flow out of the device (a phenomenon known as thermal run-off).
In another conventional PLED, a cathode may be formed over an EL layer and anode. An electrically conductive silver or nickel bonding layer having a thickness of approximately 400 nm may be formed over the cathode. An alloy foil having a thickness of approximately 200 microns is attached to the PLED by heating to melt the alloy. The foil is principally used as a substitute package to replace the packaging structures shown in FIGS. 1 and 2. The foil has a relatively low ratio of outer surface area to volume due to its substantially flat exposed upper surface (farthest from the EL layer). A thicker electrode, by itself, may not be a complete solution.
Heat dissipation issues are not unique to organic electronic devices. Inorganic (e.g., silicon-based) integrated circuits “(ICs”) can generate significant amounts of heat. Most notable are microprocessors (e.g., Intel Pentium™, AMD Athlon™, IBM PowerPC™ processors) due to their power requirements. Heat sinks can be used with inorganic ICs and typically have lengths and widths in a range of 60-100 mm and heights in a range of 30-60 mm. The ratio of the area (length times width) occupied by the inorganic IC heat sink divided by its height is in a range of 60-330. Fans may be used with the inorganic IC heat sinks. Many microprocessors have power densities of 400 W/cm2 and higher. However, many of the heat sinks used for inorganic ICs cannot be used with organic electronic devices due to materials incompatibility issues (particularly with organic layer(s)), processing issues (organic electronic devices cannot withstand higher temperature processing used with inorganic electronic devices), thickness constraints, combinations thereof, or potentially other issues.